The invention relates to correcting non-linear operation of amplifiers operating on radio frequencies. The invention particularly relates to a linearization method for an RF amplifier, in which method a digital input signal is divided into complex, baseband quadrature components employed by quadrature modulation, and the digital input signal is quadrature-modulated into a radio frequency signal and amplified.
The invention further relates to a linearized RF amplifier arrangement comprising a quadrature modulator arranged to quadrature-modulate a digital input signal into a radio frequency signal, which an amplifier is arranged to amplify.
The operating range of radio-frequency power amplifiers is wider than that of small-signal amplifiers. Hence, the power amplifiers are non-linear, which results in amplitude distortion and phase distortion. These non-linearities generate frequencies in the output signal that did not exist in the original signal. For example, a base station in a cellular radio system receives and amplifies signals of different frequencies of a plurality of terminals simultaneously, in which case, at worst, a non-linear amplifier makes a radio signal to be transmitted to spread within the frequency range to the range used by the adjacent terminal. It has been the aim of the prior art solutions to correct signal distortion caused by power amplifier intermodulation by feedforwarding or predistorting. In feedforward solutions, two control loops provided with a main amplifier and for the actual signal and a distortion amplifier for signal distortion are typically applied. Distortion feedforwarding is thus used for correcting the actual signal.
In the prior art solution using predistortion, an estimate is made of the manner in which an amplifier will distort a signal. The signal to be amplified by means of the estimate is predistorted by a distortion transformation opposite to the amplifier distortion, in other words the aim is to find out the inverse function of the distortion caused by the amplifier. The amplifier then, while amplifying the signal, simultaneously compensates for the predistortion, yielding an undisturbed, xe2x80x98linearizedxe2x80x99 signal.
In the prior art solutions, predistortion is performed either analogically or digitally. In analogue predistortion, it is difficult to detect changes in amplifier distortion; hence, digital predistortion is more preferable. Digital predistortion enables distortion to be corrected extremely efficiently. A typical digital predistortion is performed by using look-up tables that, most preferably, are to be updated in order to achieve adaptability, since amplifier distortion is affected by the amplifier""s age, temperature, and changes in the signal fed to the amplifier, for instance. U.S. Pat. No. 5,049,832, which is incorporated herein by reference, discloses such a solution. In the solution of the publication, predistortion information is stored in a memory in rectangular coordinate form, which aims to reduce the amount of information to be stored; hence, an attempt is made to quicken the adaptability of the solution in changing circumstances. Particularly in view of the algorithm, the prior art solutions of this type are not, however, sufficiently stable in the changing circumstances, which causes the amplified signal to be harmfully distorted.
An object of the invention is thus to provide a method and an arrangement implementing the method so as to solve the above problems.
This can be achieved by a method of the type described in the introduction, which is characterized by forming, from the baseband quadrature components, a difference between an output signal of the amplifier and the unpredistorted input signal supplied to the amplifier by means of an LMS algorithm or the like, forming a digital predistortion coefficient on the basis of the signal difference, and adaptively correcting the non-linearity of the amplifier before the quadrature modulation by changing the values of the complex quadrature components of the baseband input signal by means of a digital predistortion coefficient.
The amplifier arrangement of the invention is characterized in that it comprises adaptation means for forming a difference between an output signal of the amplifier and the unpredistorted input signal supplied to the amplifier from the baseband quadrature components by means of an LMS algorithm or the like, predistortion means for adaptively correcting the non-linearity of the amplifier before the quadrature modulator by changing the values of the complex quadrature components of the baseband input signal by means of a digital predistortion coefficient.
Several advantages can be achieved by the method and arrangement of the invention. Predistortion becomes more stable, and amplifier distortion is reduced.